Catalysts useful for the manufacture of maleic anhydride having a characteristic X-ray diffraction pattern

ABSTRACT

A process for the manufacture of novel phosphorus-vanadium and phosphorus-vanadium co-metal catalysts having a characteristic X-ray diffraction pattern suitable for use in the manufacture of maleic anhydride from benzene, butane and other C 4  hydrocarbons which process comprises reacting at a temperature of about 0° C. to about 200° C. a vanadium compound in an organic ether solvent having about 2 to about 10 carbon atoms with a phosphoryl halide in the presence of water or an aliphatic alcohol having from about 1 to about 8 carbon atoms, eliminating the solvent and activating the catalyst by the addition of the hydrocarbon feedstock and water and a phosphorus compound at a temperature of about 300° C. to about 500° C. wherein the amount of water added is about 1000 parts per million to about 40,000 parts per million by weight of the reactor feed gas stream is disclosed. The catalysts are useful for the manufacture of maleic anhydride from benzene, butane and other C 4  hydrocarbon feedstocks. -

This is a continuation-in-part application of Ser. No. 537,983 filed onSept. 30, 1983, now U.S. Pat. No. 4,515,904.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of this invention relates to novel catalysts and to processesfor the manufacture of phosphorus-vanadium, and phosphorus-vanadiumco-metal catalysts suitable for the oxidation of benzene, butane,butene, and butadiene to maleic anhydride.

2. Background

Maleic anhydride is of significant commercial interest throughout theworld and is extensively used in the manufacture of alkyd resins. It isalso a versatile intermediate for chemical synthesis. Consequently,large quantities of maleic anhydride are produced each year to satisfythese needs.

In general, catalysts proposed for the oxidation of butane to maleicanhydride have been based upon vanadium and phosphorus. In U.S. Pat. No.3,293,268, it is disclosed that the oxidation of butane to maleicanhydride can be performed in the presence of aphosphorus-vanadium-oxygen-containing complex catalyst. Though thiscatalyst s capable of oxidizing butane, it does not give sufficientlyhigh yields. Yields of maleic anhydride of only 30 to 50 weight percentare reported. Various activators, stabilizers and promoters have beendisclosed in the prior art to improve the yields of maleic anhydride.References include U.S. Pat. Nos. 3,867,411; 3,832,359; 3,888,886;4,002,650; 4,147,661; 4,149,992; 4,151,116; 4,152,338; 4,152,339;4,403,943; 4,154,703 and British Application 2,019,839A. While theaforementioned prior art tends to bring about some improvement in theperformance of the phosphorus-vanadium catalyst, there remains much roomfor improvement, particularly from the standpoint of high conversion,yield, and catalyst life. Other references of interest include U.S. Pat.Nos. 4,020,174; 4,094,816; 4,089,807; 3,296,282; 3,474,041 and BritishPatent 1,464,198. All of these references relate to catalystregeneration and not to new catalyst preparation.

Our catalyst prepared in an organic medium has a characteristic powderX-ray diffraction pattern using copper K alpha radiation as follows:

    ______________________________________                                                      Line Position                                                                            Relative                                             d, angstrom   2θ degrees                                                                         Intensity                                            ______________________________________                                        5.71          15.52      37                                                   4.80          18.46       8                                                   4.52          19.64      24                                                   3.67          24.21      18                                                   3.29          27.06      25                                                   3.11          28.67      12                                                   2.94          30.41      100                                                  2.79          32.00      12                                                   2.65          33.81       7                                                   2.61          34.28       7                                                   2.40          37.39       12.                                                 ______________________________________                                    

Our catalyst is suitably prepared in organic solvents by slurryingvanadium compounds and metals or metal oxides such as molybdenum oxide,zinc oxide, uranium oxide, tungsten oxide, tin oxide, bismuth oxide,titanium oxide, niobium oxide, antimony oxide and cobalt oxide inorganic solvents, preferably organic ether solvents.

A small amount of water or a hydrogen donor compound, such as a loweralcohol, is also present in the ether. Suitable alcohols are ethanol andmethanol and suitable ethers are tetrahydrofuran (THF), tetrahydropyran,1,2-dimethoxyethane, bis(2-methoxyethyl)ether, 1,4-dioxane, ethylether,propylether, butylether, and pentylether. Phosphoryl halide is slowlyadded to the slurry. The water or hydrogen donor reacts with thephosphoryl halide to generate anhydrous phosphoric acid or phosphateesters and hydrogen halide gas. The hydrogen halide dissolves both thevanadium compound, for example, the vanadium pentoxide, and the co-metalcompound and also reduces the vanadium from a valence state of aboutfive to a valence state of about four. This reaction takes place at atemperature of about 0° C. to about 200° C.

While the reaction solution is being refluxed, if desired, a modifier ormixture of modifiers such as o-xylene, m-xylene, p-xylene, benzene,toluene, mesitylene, pseudocumene, phthalic anhydride, trimelliticanhydride, benzoic acid, toluic acid, phthalic acid, isophthalic acid,terephthalic acid, trimesic acid or trimellitic acid, is suitably addedto the reaction solvent. After refluxing, the color of the solution isgreen. The volume of the solution is reduced by distillation orevaporation until it becomes a thick syrup. This syrup is dried at atemperature of about 120° C. to about 150° C. and 0-15 inches of mercuryvacuum under an air purge. Once dry, the color of the solid material isbrown. The catalyst can be formed into geometric forms, such ascylinders, using graphite, Sterotex or other lubricants such as stearicacid, zinc stearate or starch and binders such as polyvinyl alcohol. Thecatalyst in the form of geometric shapes or in powder form is suitablycalcined in air or a nitrogen-air combination before loading into asuitable tubular reactor. The catalyst is activated further by theaddition of water and phosphorus compounds or mixtures thereof such asalkylphosphates, phosphites, and phosphines. This activation takes placeat a temperature of about 300° C. to about 500° C. Representativephosphorus compounds have the following structure: ##STR1## wherein R isphenyl or an alkyl radical of 1 to 6 carbon atoms and X is H or R.Suitable compounds are primary, RPH₂, secondary, R₂ PH, and tertiary, R₃P, phosphines, such as ethyl phosphine; the tertiary phosphine oxides,R₃ PO, such as tripropyl phosphine oxide; the primary, RP(O)(OX)₂, andsecondary, R₂ P(O)OX, phosphonic acids, such as benzene phosphonic acid;the esters of the phosphonic acids, such as diethyl methanephosphonate;the phosphonous acids, RPO₂ X₂, such as benzenephosphonous acid and theesters thereof, such as the monoethyl ester; the phosphinous acids, R₂POX, such as diethyl phosphinous acid and the esters thereof, such asthe monoethyl ester; the primary, ROP(OX)₂, secondary, (RO)₂ POX, andtertiary, (RO)₃ P, phosphites, such as diethyl phosphite, trimethylphosphite, triethyl phosphite, triisopropyl phosphite, tripropylphosphite and tributyl phosphite, and the pyrophosphites, such astetraethyl pyrophosphite. The preferred phosphorus compound is an esterof orthophosphoric acid having the formula (RO)₃ P=O wherein R ishydrogen or a C₁ -C₄ alkyl, at least one R being a C₁ -C₄ alkyl. Thepreferred phosphorus compounds are triethylphosphate andtrimethylphosphate.

Our catalyst has a much higher yield of maleic anhydride from butanefeedstock than catalysts of the prior art, such as those disclosed inU.S. Pat. No. 3,862,146, and U.S. Pat. No. 4,328,126. Among the manyadvantages of our novel process for the manufacture of the catalyst canbe cited the quantitative use of the expensive vanadium and the use ofphosphoryl halides as a source of phosphorus and inexpensive solventssuch as organic ethers in combination with small amounts of water ormethanol or ethanol.

The novel catalyst comprises a phosphorus-vanadium mixed oxide or aphosphorus-vanadium mixed oxide promoted by metals. The atomic ratio ofthe vanadium to phosphorus can suitably be in the range of 0.5:1 to1.25:1.0. The total atomic ratio of vanadium to phosphorusadvantageously is in the range of 0.75:1 to 1:1. It is preferred thatthe total atomic ratio of molybdenum, zinc, tungsten, uranium, tin,bismuth, titanium, niobium or cobalt to vanadium should be in the rangeof 0.001:1 to 0.2:1. The atomic ratio of phosphorus to vanadium issuitably in the range of 0.8:1 to 2:1, preferably 1:1 to 1.3:1.

The co-metal, such as molybdenum, zinc, tungsten, uranium, bismuth,titanium, antimony, niobium, cobalt or tin may be added as a compoundtogether with vanadium, or separately introduced into the solution.Suitable cometal compounds comprise their oxides and soluble salts.Suitable molybdenum compounds comprise molybdenum oxide and most solublemolybdenum salts. If it is desired to improve physical properties of thecatalysts, they may be treated with the suspension of an inert support,for example, alumina, titania, silicon carbide, kieselguhr, pumice orsilica. The catalyst may be reinforced with such materials at any stagein its preparation.

It has been discovered that the catalyst has a characteristic X-raydiffraction pattern as follows:

    ______________________________________                                                      Line Position                                                                            Relative                                             d, angstrom   2θ degrees                                                                         Intensity                                            ______________________________________                                        5.71          15.52      37                                                   4.80          18.46       8                                                   4.52          19.64      24                                                   3.67          24.21      18                                                   3.29          27.06      25                                                   3.11          28.67      12                                                   2.94          30.41      100                                                  2.79          32.00      12                                                   2.65          33.81       7                                                   2.61          34.28       7                                                   2.40          37.39       12.                                                 ______________________________________                                    

According to our process, the average valence of vanadium is in therange of about 3.8 to 4.2. In our catalyst preparation, variousphosphoryl halides may be used, but POCl₃ is preferred. The catalyst canbe activated with water and: ##STR2## wherein R is phenyl or an alkylradical of 1 to 6 carbon atoms and X is H or R. Suitable compounds arethe primary, RPH₂, secondary, R₂ PH, and tertiary, R₃ P, phosphines,such as ethyl phosphine; the tertiary phosphine oxides, R₃ PO, such astripropyl phosphine oxide; the primary, RP(O)(OX)₂, and secondary, R₂P(O)OX, phosphonic acids, such as benzene phosphonic acid; the esters ofthe phosphonic acids, such as diethyl methanephosphonate; thephosphonous acids, RPO₂ X₂, such as benzenephosphonous acid and theesters thereof such as the monoethyl ester; the phosphinous acids, R₂POX, such as diethyl phosphinous acid and the esters thereof, such asthe monoethyl ester; the primary, ROP(OX)₂, secondary, (RO)₂ POX, andtertiary, (RO)₃ P, phosphites, such as diethyl phosphite, trimethylphosphite, triethyl phosphite, triisopropyl phosphite, tripropylphosphite and tributyl phosphite, and the pyrophosphites, such astetraethyl pyrophosphite. The preferred phosphorus compound is an esterof orthophosphoric acid having the formula (RO)₃ P=O wherein R ishydrogen or a C₁ -C₄ alkyl, at least one R being a C₁ -C₄ alkyl. Thepreferred phosphate compounds are triethylphosphate andtrimethylphosphate.

The amount of water added is about 1000 to about 40,000 parts permillion of the reaction feed gas stream. The reaction feed gas streamcomprises hydrocarbon and air.

Suitable vanadium compounds include: vanadium oxides, such as vanadiumpentoxide, vanadium trioxide and the like; vanadium oxyhalides, such asvanadyl chloride, vanadyl dichloride, vanadyl trichloride, vanadylbromide, vanadyl dibromide, vanadyl tribromide and the like;vanadium-containing acids, such as meta-vanadic acid, pyrovanadic acidand the like; vanadium salts, such as ammonium meta-vanadate, vanadiumsulfate, vanadium phosphate, vanadyl formate, vanadyl oxalate and thelike; however, vanadium pentoxide is preferred.

This invention also comprises a process for oxidizing butane to maleicanhydride by contacting it in the presence of oxygen with the novelcatalyst. The oxidation of butane to maleic anhydride may beaccomplished by contacting n-butane in low concentration in oxygen withthe described catalyst. Air is entirely satisfactory as a source ofoxygen, but synthetic manufactures of oxygen and diluent gases, such asnitrogen also may be employed. Air enriched with oxygen may be used.

The gaseous feed stream to the oxidation reactors will normally containair and about 0.2 to about 1.7 mole percent of n-butane. About 0.8 to1.5 mole percent of n-butane is satisfactory for optimum yield of maleicanhydride for the process of this invention. Although higherconcentrations may be employed, explosive hazards may be encountered.Lower concentrations of butane, less than about one percent, of course,will reduce the total yield obtained at equivalent flow rates and, thus,are not normally economically employed. The flow rate of the gaseousstream through the reactor may be varied within rather wide limits, butpreferred range of operations is at the rate of about 100 to 4000 cc offeed per cc of catalyst per hour, and more preferably about 1000 to 2400cc of feed per cc of catalyst per hour. Residence times of the gasstream will normally be less than about four seconds, more preferablyless than about one second, and down to a rate where less efficientoperations are obtained. The flow rates and residence times arecalculated at standard conditions of 760 mm of mercury at 0° C. Avariety of reactors will be found to be useful, and multiple tube heatexchanger-type reactors are quite satisfactory. The tops of suchreactors may vary in diameter from about one-quarter inch to about threeinches, and the length may be varied from about three to about ten ormore feet. The oxidation reaction is an exothermic reaction and,therefore, relatively close control of the reaction temperatures shouldbe maintained. It is desirable to have the surface of the reactors atrelatively constant temperatures, and some medium to conduct heat fromthe reactors is necessary to aid temperature control. Such media may beWoods metal, molten sulfur, mercury, molten lead and the like, but ithas been found that eutectic salt baths are completely satisfactory. Onesuch salt bath is a sodium nitrate-sodium nitrite-potassium nitrateeutectic constant temperature mixture. An additional method oftemperature control is to use a metal block reactor whereby the metalsurrounding the tube acts as a temperature-regulating body. As will berecognized by one skilled in the art, the heat exchanger medium may bekept at the proper temperature by heat exchangers and the like. Thereactor or reaction tubes may be iron, stainless steel, carbon steel,nickel, glass tubes such as vycor, and the like. Both carbon steel andnickel tubes have excellent long life under the conditions of thereaction described herein. Normally, the reactors contain a preheat zoneunder an inert material such as one-quarter-inch alundum pellets, inertceramic balls, nickel balls, or chips and the like present at aboutone-half to one-tenth the volume of the active catalyst present.

The temperature of reaction may be varied within some limits, butnormally the reaction should be conducted at a temperature within arather critical range. The oxidation reaction is exothermic and oncereaction is underway, the main purpose of the salt bath or other mediais to conduct heat away from the walls of the reactor and control thereaction. Better operations are normally obtained when the reactiontemperature employed is no greater than 20°-50° F. above the salt bathtemperature. The temperature of the reactor, of course, will also dependto some extent upon the size of the reactor and the butaneconcentration.

The reaction may be conducted at atmospheric, superatmospheric, orsubatmospheric pressure. The exit pressure will be at least slightlyhigher than the ambient pressure to ensure a positive flow from thereactor. The pressure of the inert gases must be sufficiently high toovercome the pressure drop through the reactor.

Maleic anhydride may be recovered by a number of ways well-known tothose skilled in the art. For example, the recovery may be by directcondensation or by absorption in suitable media, with specificoperations and purification of the maleic anhydride. The followingexamples will serve to provide full understanding of the invention, butit is to be understood that these examples are given for illustrativepurposes only and will not be interpreted as limiting the invention inany way. In the Examples the terms "conversion", "selectivity" and"yield" are defined as follows: ##EQU1##

EXAMPLE 1

To a 3-liter, 3-neck, round-bottom flask equipped with a thermowell,electrical mantle, mechanical stirrer, and reflux condenser, were added91 g of V₂ O₅, 4.4 g of MoO₃, 196.3 g of POCl₃, and 500 ml oftetrahydrofuran (THF). Water, 69 g, was slowly added from a droppingfunnel to the slurry causing the V₂ O₅ to dissolve and the solvent toreflux. The red-brown solution was refluxed at 103° C. for 13.5 hoursreducing the vanadium (V) to vanadium (IV) and turning the color togreen.

The solvent was distilled from the green solution until a thick syrupremained. The syrup was put in a vacuum oven overnight at 10 in. of Hgvacuum and 130° C. with a slight air purge passing through the oven. Thedry catalyst precursor which was brown in color was ground and formedinto 3/16" cylindrical tablets using 5 wt. % graphite as a lubricant.The side crush strength of the tablets was 6.5-7.5 lbs.

A 6 cm³ charge of the tablets was loaded into a 0.62" diameterminireactor and evaluated with a feed of 1.1% n-butane in synthetic airat 1200 VHSV. About 10,000 ppm of water were continually added to thereactor feedstream by passing it through a water saturator. Thiscatalyst gave a maximum maleic anhydride yield of 103 wt. % at atemperature of 741° F. after 53 days on stream. The conversion was 89mole % and the selectivity 69 mole %. The surface area of this catalystwas 40 m² /g and the pore volume by adsorption was 0.215 cc/g.

EXAMPLE 2

A catalyst precursor was prepared in a similar manner as described inExample 1 except that the solution was refluxed for 11.5 hours. Thedried powder was ground, mixed with 5 wt. % graphite, and formed into3/16" cylinders having a 6.5-8.0 lb. side crush strength. These tabletswere calcined to 700° F. in air before being charged into a minireactorand evaluated as reported in Example 1. This catalyst gave a maximummaleic anhydride yield of 102 wt. % at 745° F. The conversion was 90mole % and the selectivity 67 mole %.

EXAMPLE 3

A catalyst precursor was prepared as reported in Example 1 except thatthe solution was refluxed for only 5 hours and 184 g of POCl₃ were usedgiving a 1.2/1 P/V ratio instead of 1.28/1 as used in Examples 1 and 2.The powder was mixed with 5 wt. % graphite and formed into 3/16"cylinders having a 6-7 lb. side crush strength.

The catalyst was evaluated in a minireactor as described in Example 1.The catalyst gave a maximum maleic anhydride yield of 100 wt. % at 753°F. with an 89 mole % conversion and a 66 mole % selectivity.

EXAMPLE 4

A catalyst precursor was prepared in a similar manner as in Example 1.Phthalic anhydride, 22 g, was added to the solution which was refluxedfor 12.5 hours. The powder was mixed with 5 wt. % graphite and formedinto 3/16" cylindrical tablets having a 6-7.5 lb. side crush strength.

The catalyst was evaluated in a minireactor as described in Example 1. Amaximum maleic anhydride yield of 100 wt. % was obtained at 741° F. Theconversion was 85 mole % and the selectivity 70 mole %. The catalystsurface area was 42 m² /g and the pore volume by adsorption was 0.2787cc/g.

EXAMPLE 5

A 12-liter, 3-neck, round-bottom flask equipped with an electricalmantle, mechanical stirrer, thermowell, and reflux condenser, wascharged with 2 liters of THF, 364 g of V₂ O₅, 17.6 g of MoO₃, and 767 gof POCl₃. Water, 270 g, was added slowly causing the V₂ O₅ to dissolveand turning the solution red-brown. The solution was refluxed for 14.5hours with the color changing from red-brown to green. Solvent wasremoved by distillation until the temperature reached 129° C. The thicksyrup was put into a vacuum oven at 3-5 in. Hg vacuum and 130° C. with aslight air purge passing through the oven. The dry material was ground,mixed with 5 wt. % graphite, and formed into 3/16" cylindrical tabletshaving a 5-15 lb. side crush strength. The tablets were calcined in airto 371° C.

This catalyst, 120 g, was loaded into a pilot plant having a 0.62"diameter minireactor. After 1219 hours on stream, the catalyst gave amaximum maleic anhydride yield of 77 wt. % at 1.5% n-butane in air feed,2000 VHSV, and 707° F. salt bath temperature. The feed stream was thenpassed through a saturator containing a solution of 0.4 gtriethylphosphate per liter of water. In this manner, about 10,000 ppmof water were added to the feed stream. The catalyst gave a maximummaleic anhydride yield of 87 wt. % at the same conditions and a saltbath temperature of 725° F. after 1723 hours on stream. Thus, theaddition of water and triethylphosphate in small quantities to the feedstream resulted in improving the catalyst yield by 10 wt. %. Theconcentration of the saturator solution was increased to 0.7 gtriethylphosphate per liter of water. After 2395 hours on stream, theyield of maleic anhydride was 89 wt. % at the same conditions and a saltbath temperature of 722° F.

EXAMPLE 6

Using the same experimental setup as described in Example 5, 2 liters ofTHF, 364 g of V₂ O₅, 17.3 g of MoO₃, and 270 g of water were charged tothe large flask. POCl₃, 767 g, was added slowly to the mixture causingthe V₂ O₅ to dissolve and turning the color of the solution tored-brown. At this time 500 ml of o-xylene were added to the solutionand it was refluxed for 16.5 hours causing the solution color to changeto green. The solvent was then removed by distillation until thetemperature of the catalyst syrup reached 139° C. The syrup was placedinto a vacuum oven overnight at 150° C. and 5 in. of Hg vacuum with aslight air purge passing through the oven. The dry brown material wasground, mixed with 5 wt. % graphite, and formed into 3/16" cylindricaltablets having a 3 lb. side crush strength.

A 6 cm³ charge of the tablets was evaluated in a minireactor asdescribed in Example 1. This catalyst gave a maximum maleic anhydrideyield of 101 wt. % at 778° F. after 22 days on stream. The conversionwas 90 mole % and the selectivity was 66.5 mole %.

This example illustrates that an excellent catalyst can be prepared byadding POCl₃ to the reaction mixture which contains a hydrogen donorcompound. This method of addition gives better temperature control ofthe exothermic reaction; also, the use of the modifier o-xylene, ahigher distillation temperature, and a longer reflux time aredemonstrated in this Example.

EXAMPLE 7

Using the experimental setup described in Example 1, 91 g of V₂ O₅, 4.3g of MoO₃, 184 g of POCl₃, and 500 ml of THF were charged to thereaction flask. Water, 65 g, was slowly added in 1 hour to the flaskcausing the vanadium to dissolve and form a red-brown solution.O-xylene, 150 ml, was added to the reaction solution and it was refluxedfor 20.5 hours. Solvent was removed by distillation until thetemperature of the catalyst syrup reached 151° C. The syrup was driedovernight in a vacuum oven at 140° C. at 0 in. of Hg vacuum with an airpurge passing through the oven.

The dried material was ground, mixed with 5 wt. % graphite, and formedinto 3/16" cylinders having a 3 lb. side crush strength. The catalystwas evaluated as described in Example 1. The catalyst having a P/V ratioof 1.2/1 gave a maximum maleic anhydride yield of 100 wt. % at 727° F.after 42 days on stream.

EXAMPLE 8

A catalyst was prepared and evaluated as reported in Example 7 exceptthat 11.8 g of zinc metal was used in the catalyst preparation insteadof MoO₃. This catalyst gave a maximum maleic anhydride yield of 86 wt. %at 822° F. after 50 days on stream.

EXAMPLE 9

A catalyst was prepared and evaluated as described in Example 7 exceptthat 6.96 g of WO₃ instead of MoO₃ were used in the preparation and thePOCl₃ was added to the reaction mixture containing the water. Thiscatalyst gave a maximum maleic anhydride yield of 88 wt. % at 736° F.after 49 days on stream.

EXAMPLE 10

A catalyst was prepared and evaluated as described in Example 9 exceptthat 23.19 g of WO₃ was used in the preparation. The catalyst gave amaximum maleic anhydride yield of 83 wt. % at 763° F. after 21 days onstream.

EXAMPLE 11

A catalyst was prepared and evaluated as reported in Example 9 exceptthat Sn metal, 3.56 g, was used in place of WO₃. The catalyst gave amaximum maleic anhydride yield of 81 wt. % at 761° F. after 29 days onstream.

EXAMPLE 12

A catalyst was prepared and evaluated as described in Example 9 exceptthat Co₂ O₃, 2.49 g, was used in place of WO₃. The catalyst gave amaximum maleic anhydride yield of 87 wt. % at 759° F. after 30 days onstream.

Examples 9-12 show that phosphorus-vanadium catalysts prepared withco-metals other than molybdenum will give excellent yields of maleicanhydride.

EXAMPLE 13

After the catalyst in Example 9 was on stream for 55 days, the feed gaswas passed through a saturator containing a 20 wt. % aqueous solution oftriethylphosphate. The yield from this catalyst increased to 92 wt. % at737° F. 10 days later. This treatment improved the yield of thiscatalyst by 4 wt. %.

EXAMPLE 14

After 27 days on stream the feed gas to the catalyst in Example 10 waspassed through a saturator containing a 1-5 wt. % aqueous solution oftriethylphosphate. The yield increased to 97 wt. % at 752° F. 18 dayslater which is a 14 wt. % improvement in the yield of this catalyst.

EXAMPLE 15

The feed gas to the catalyst in Example 12 was passed through asaturator containing a 1-5 wt. % aqueous solution of triethylphosphateafter 31 days on stream. The yield increased to 90 wt. % at 759° F. 7days later for an improvement in yield of 3 wt. %.

Examples 13-15 illustrate that phosphorus-vanadium catalysts havingco-metals other than molybdenum, and prepared according this invention,will give improved yields when treated in situ with phosphorus compoundsand water.

EXAMPLE 16

A phosphorus-vanadium-molybdenum oxide catalyst having a P/V/Mo atomicratio of 1.1/1/0.03 was prepared according to the method reported inU.S. Pat. No. 4,416,803. Prior to activation in n-butane and air, theoxide catalyst was analyzed by X-ray diffraction and gave the followingcharacteristic powder diffraction pattern:

    ______________________________________                                                      Line Position                                                                            Relative                                             d, angstrom   2θ degrees                                                                         Intensity                                            ______________________________________                                        5.72          15.47      100                                                  4.81          18.43       5                                                   4.53          19.56      39                                                   3.68          24.16      31                                                   3.30          27.02      39                                                   3.12          28.63      17                                                   2.94          30.38      54                                                   2.80          31.99      14                                                   2.67          33.60      12                                                   2.61          34.28       7                                                   2.40          37.36        7.                                                 ______________________________________                                    

The crystallinity of this catalyst powder was at least 75% with theremainder being amorphous.

The oxide catalyst was ground, mixed with 5 wt. % graphite, and formedinto 3/16" by 3/16" cylindrical tablets having a 4 lb. side crushstrength. The catalyst, 5.39 g, was evaluated as described in Example 1.The catalyst gave a maximum maleic anhydride yeild of 93 wt. % after 16days on stream at 824° F. and with 85% conversion.

EXAMPLE 17

A catalyst was prepared as described in Example 6 except that thesolution was refluxed for 5 hours and the syrup was dried at 140° C. and4 in. of Hg vacuum. This powder gave the following X-ray diffractionpattern:

    ______________________________________                                                      Line Position                                                                            Relative                                             d, angstrom   2θ degrees                                                                         Intensity                                            ______________________________________                                        5.71          15.52      37                                                   4.80          18.46       8                                                   4.52          19.64      24                                                   3.67          24.21      18                                                   3.29          27.06      25                                                   3.11          28.67      12                                                   2.94          30.41      100                                                  2.79          32.00      12                                                   2.65          33.81       7                                                   2.61          34.28       7                                                   2.40          37.39       12.                                                 ______________________________________                                    

Although the d spacings are essentially the same as reported in Example16, the lines are much broader in this diffraction pattern and therelative intensities of the lines are very different. The crystallinityof the powder was at least 68% with the remainder being amorphous.

The dried material was crushed and calcined in air to 371° C. Thiscalcined powder gave the same X-ray diffraction pattern but thecrystallinity had declined to 24%. The calcined powder was mixed with 5wt. % graphite and formed into 3/16" by 3/16" cylindrical pellets havinga 3-6 lb. side crush strength. A 6 cm³ load, 5.20 g, of this catalystwas evaluated in a minireactor as described in Example 1. After 16 dayson stream the maleic anhydride yield from this catalyst was 94 wt. % at776° F. with 87% conversion. The maximum yield of 100 wt. % was achievedafter 36 days on stream at 771° F. with 91% conversion.

This catalyst having the above characteristic X-ray diffraction patternis clearly superior in performance to the catalyst in Example 16.

EXAMPLE 18

A catalyst was prepared as described in Example 6. This material gavethe same characteristic X-ray diffraction pattern as reported in Example17. The dried catalyst was crushed and calcined in air to 371° C. TheX-ray diffraction pattern of this calcined material was the same as theuncalcined material.

The calcined powder was mixed with 5 wt. % graphite and formed into3/16" by 3/16" cylindrical tablets having a 5-6 lb. side crush strength.This catalyst, 136 g, was charged to a pilot plant having a 0.62"internal diameter reactor. The performance of this catalyst at 1.5%n-butane in air feed and 2000 VHSV is summarized in Table I.

                  TABLE I                                                         ______________________________________                                        Performance of Catalyst from Example 18                                       Hours Salt    Triethylphos-                                                   on    Temp.,  phate in the                                                                              Water in the                                                                           Conv.,                                                                              Yield,                               Stream                                                                              °F.                                                                            feed, ppm   feed, ppm                                                                              %     Wt. %                                ______________________________________                                         450  777     0              0     81    78                                    642  --      5           10,000   --    --                                    786  --      10          10,000   --    --                                    858  742     10          10,000   80    80                                   1050  746     10          10,000   76    84                                   1146  750     10          10,000   82    86                                   1170  --      15          10,000   --    --                                   1386  --      17.5        10,000   --    --                                   1674  777     17.5        10,000   83    88                                   ______________________________________                                    

After 450 hours on stream, the catalyst gave a maximum maleic anhydrideyield of 78 wt. % (81% conversion) at 777° F., 2000 VHSV, and 1.5 mole %n-butane in air feed. Using an Isco metering pump 10,000 ppm of waterand 5 ppm of triethylphosphate were added to the reactor feed at 642hours. The triethylphosphate concentration was increased to 10 ppm at786 hours. At 858 hours, the yield improved to 80 wt. % and to 86 wt. %after 1146 hours. A further increase in triethylphosphate to 17.5 ppm at1386 hours produced an 88 wt. % yield at 1674 hours on stream with 83%conversion, 2000 VHSV, 1.5 mole % n-butane in air feed, and 777° F. salttemperature.

This Example illustrates that activation of this catalyst withtriethylphosphate and water added to the n-butane and air feed producesa 10 wt. % improvement in maleic anhydride yield.

EXAMPLE 19

A catalyst was prepared according to the process in Example 6. Thecharacteristic X-ray diffraction pattern of the dried catalyst powderwas the same as the pattern in Example 17. The dried material wascrushed and calcined in air to 371° C. The X-ray diffraction pattern ofthis powder was the same as the uncalcined material.

The calcined powder was mixed with 5 wt. % graphite and formed into3/16" by 3/16" cylindrical tablets having a 4-6 lb. side crush strength.This tableted catalyst, 138 g, was charged to a pilot plant having a0.62" internal diameter reactor. The performance of this catalyst at1.5% n-butane in the feed and 2000 VHSV is summarized in Table II.

                  TABLE II                                                        ______________________________________                                        Performance of Catalyst from Example 19                                       Hours Salt    Triethylphos-                                                   on    Temp.,  phate in the                                                                              Water in the                                                                           Conv.,                                                                              Yield,                               Stream                                                                              °F.                                                                            feed, ppm   feed, ppm                                                                              %     Wt. %                                ______________________________________                                         262  772     0              0     77    77                                    740  --      10             0     --    --                                   1004  751     10             0     81    73                                   1028  --      10          10,000   --    --                                   1076  762     10          10,000   77    83                                   1244  763     10          10,000   82    87                                   1412  --      7           10,000   --    --                                   1508  754     7           10,000   81    88                                   1772  --      8           10,000   --    --                                   1844  760     8           10,000   83    85                                   1868  --      0           10,000   --    --                                   2272  721     0           10,000   81    74                                   ______________________________________                                    

The catalyst gave a maximum maleic anhydride yield of 77 wt. % (77%conversion) after 262 hours on stream with no water andtriethylphosphate addition to the feed. At 740 hours, 10 ppm oftriethylphosphate were added to the feed using an Isco metering pumpkeeping the VHSV at 2000 and 1.5% n-butane in the feed. In this case,the catalyst yield did not increase as occurred in Example 18 when bothtriethylphosphate and water were added to the feed during activation. At1004 hours, the yield was only 73 wt. % despite the addition of 10 ppmof triethylphosphate to the feed for 364 hours. Water, 10,000 ppm, wasthen introduced into the feed with an Isco metering pump along with 10ppm of triethylphosphate at 1028 hours. Only 48 hours at theseconditions produced an 83 wt. % yield of maleic anhydride. An 88 wt. %yield of maleic anhydride was achieved at 1508 hours with 7 ppm oftriethylphosphate and 10,000 ppm of water being added to the feed. Thetriethylphosphate was removed from the feed at 1868 hours. By 2272hours, the yield had declined to 74 wt. %.

This Example shows that the addition of both triethylphosphate and waterto the feed during catalyst activation is required to significantlyimprove the catalyst yield. Catalyst activation with n-butane and airfeed and either triethylphosphate or water alone does not significantlyimprove the catalyst yield.

EXAMPLE 20

Using the same experimental setup described in Example 5, 1 liter ofTHF, 364 g of V₂ O₅, 17.3 g of MoO₃, and 270 g of water were charged tothe large flask. POCl₃, 767 g, was added dropwise from an additionfunnel over a 2.5-hour time period causing the V₂ O₅ to dissolve andturning the solution red-brown. The contents of the flask were allowedto reflux for 2 hours following the end of the POCl₃ addition. Thetemperature of the flask contents during reflux was 93°-111° C. At thistime, about 700 ml of solvent were removed by distillation leaving adark green syrup. The syrup was dried in an air-ventilated oven at 148°C. and 1-5 in. of Hg vacuum. The dried material gave the samecharacteristic X-ray diffraction pattern reported in Example 17. Thedried brown material was crushed and calcined in air to 371° C. Thecalcined material gave the same characteristic X-ray diffractionpattern.

The calcined powder was mixed with 5 wt. % graphite and formed into3/16" by 3/16" cylindrical tablets having a 5 lb. side crush strength.These tablets, 130 g, were charged to a pilot plant having a 0.62"internal diameter reactor. The performance of this catalyst at 1.5%n-butane in air feed and 2000 VHSV is reported in Table III.

                  TABLE III                                                       ______________________________________                                        Performance of Catalyst from Example 20                                       Hours Salt    Triethylphos-                                                   on    Temp.,  phate in the                                                                              Water in the                                                                           Conv.,                                                                              Yield,                               Stream                                                                              °F.                                                                            feed, ppm   feed, ppm                                                                              %     Wt. %                                ______________________________________                                        235   768     0              0     81    81                                   379   --      5           10,000   --    --                                   591   742     5           10,000   80    83                                   783   750     5           10,000   84    89                                   903   754     5           10,000   81    89                                   ______________________________________                                    

The catalyst gave a maximum yield of 81 wt. % (81% conversion) at 768°F. with no triethylphosphate or water addition to the feed after 235hours on stream. Using an Isco metering pump, 5 ppm of triethylphosphateand 10,000 ppm of water were added to the feed at 379 hours. At 591hours, the maleic anhydride yield was 83 wt. % improving to 89 wt. % at783 hours.

This Example indicates that an excellent catalyst having thecharacteristic X-ray diffraction pattern can be prepared with a lowersolvent-to-vanadium ratio, no o-xylene, and a 2-hour reflux of thereaction mixture.

I claim:
 1. A catalyst for the production of maleic anhydrice by theoxidation of butane which comprises a phosphorus-vanadium mixed oxide,the atomic ratio of vanadium to phosphorus being in the range of 0.5:1to 1.25:1 wherein the catalyst has a characteristic powder X-raydiffraction pattern using copper K alpha radiation as follows:

    ______________________________________                                                      Line Position                                                                            Relative                                             d, angstrom   2θ degrees                                                                         Intensity                                            ______________________________________                                        5.71          15.52      37                                                   4.80          18.46       8                                                   4.52          19.64      24                                                   3.67          24.21      18                                                   3.29          27.06      25                                                   3.11          28.67      12                                                   2.94          30.41      100                                                  2.79          32.00      12                                                   2.65          33.81       7                                                   2.61          34.28       7                                                   2.40          37.39      
 12.                                                 ______________________________________                                    


2. The catalyst of claim 1, wherein there are 0.8 to 2 atoms ofphosphorus present for each atom of vanadium.
 3. The catalyst of claim 1wherein a co-metal is used as a promoter wherein the total atomic ratioof the co-metal to vanadium is in the range of 0.001:1 to 0.4:1.
 4. Thecatalyst of claim 3 wherein the co-metal is molybdenum.
 5. The catalystof claim 3 wherein the co-metal is zinc.